1. Field of the Invention
The present invention relates to an RF switch and a process for making an RF switch and more particularly, to an RF switch fabricated by way of microelectromechanical system (MEMS) technology which includes a planar airbridge which allows for switch deflection in a single plane generally parallel to the substrate and thus only requires a single level of metallization, greatly simplifying the fabrication of the switch relative to known switches.
2. Description of the Prior Art
RF switches are used in a wide variety of applications. For example, such RF switches are known to be used in variable RF phase shifters; RF signal switching arrays; switchable tuning elements as well as in gang switching of voltage control oscillators (VCO). In order to reduce the size and weight of such RF switches, microelectromechanical system (MEMS) technology has been known to be used to fabricate such switches. MEMS technology is a process for fabricating various components using micromaching in a very similar manner as integrated circuits are fabricated.
Switches fabricated using MEMS technology normally include a substrate with one or more metal traces and control pads. An airbridged beam is known to be formed over the substrate in order to form one or more contacts with one or more of the metal traces; however, with only a single throw. Such switches normally require multiple levels of metallization.
Electrostatic forces are known to be used to control the opening and closing of the contacts. In particular, the control pad is connected to an external source of DC voltage. When the DC voltage is applied to the control contact, electrostatic forces cause the beam to deflect and make contact with one of the contacts, thus closing the circuit between the metal trace and the beam which define an RF contact. When the DC voltage is removed from the control pad, in some known switches, the resiliency of the beam causes it to deflect back to its normal position. In other known switches, electrostatic force is required to return the beam to the normal position. With such switches, the deflection of the beam is normally in a plane generally perpendicular to the plane of the substrate.
U.S. Pat. No. 5,619,061 and in particular FIGS. 18A-18D of the '061 patent discloses an RF switch with a single pole configuration, formed from multiple levels of metallization. In particular, the '061 patent discloses an RF switch which includes a beam suspended on opposing edges by thin metal hinges. More particularly, the beam is spaced apart from the substrate and suspended about midway along each edge by way of thin metal hinges. Metal traces are applied to the substrate and aligned with the edges of the beam. Control pads are disposed on the substrate adjacent the metal traces. Application of a DC voltage to the control pads causes an electrostatic attraction force to rotate the beam clockwise or counter clockwise and make contact with one of the metal traces on the substrate.
There are several known disadvantages of such RF switches. For example, such switches require a minimum of two levels of metal deposition, which adds to the complexity of the fabrication process. In addition, such switches are known to require relatively high voltages, typically 20-30 volts to operate. The relatively high voltage requirement is due to either the limited length of the airbridge, limited because of the possibility of collapsing, or due to the large distance between the beam and the DC control pad. Because of the possibility of foreign particles getting underneath the metal flap or membrane, such switches are normally limited to single throw designs because more throws normally require additional complicated metal deposition steps which could collapse onto lower levels. In addition, one of the failure mode for these kinds of switch is so called "sticking on", the switches stay at "on" position permanently. Thus, there is a need to provide an RF switch which has multiple throws that is amenable to being fabricated using MEMS technology which is less complicated to fabricate, remedy "sticking on" problem, and only requires a single level of metallization.